It isn't the hubs fault, it's mostly the USB consortium lying about the real world practical throughput of their technology for marketing purposes. Tom's HW has a good technical writeup on it, it's from 2012 but most of it still applies: http://www.tomshardware.com/reviews/usb-3-uas-turbo,3215.html
Ultimately you want no more than 3 drives per USB controller. PCI-e cards like this are ideal, as they contain 4 separate buses (allowing 12 total drives at ~120 MB speeds) https://www.amazon.com/Express-SuperSpeed-Adapter-Dedicated-Channels/dp/B00HJZEA2S/ref=sr_1_1
If it were me, for a core system I'd be looking at something like this: https://pcpartpicker.com/list/W9mvKB
That CPU supports avx2 which should allow good plotting speed. Also has hyperthreading meaning it can read multiple drives at once decreasing overall read speed.
We're going to use an nvme drive to stage plots using turboplotter because the write speed on those Seagate externals is completely terrible if you try plotting them directly.
Finally, with a built-in GPU, you won't need to purchase a discreet unit allowing you to spend a few extra dollars on 16gigs of fast RAM (ryzen specifically needs fast RAM). Now because we won't be using our pcie slots on unnecessary GPU's, you can add two USB host controller cards like this one: https://www.amazon.com/gp/aw/d/B00HJZEA2S/ref=mp_s_a_1_2?ie=UTF8&qid=1519885103&sr=8-2&pi=AC_SX236_SY340_FMwebp_QL65&keywords=startech+usb+pcie as to not overburden the onboard USB controller.
Please note however, the ryzen 5 2400g is a new offering from AMD so please please contact your mobo vendor or ASRock and confirm that you can install this specific apu without requiring a BIOS flash before it'll function (having to do so would mean temporarily installing an older chip).
Hope this helps.
I got it from eBay. Something like this listing for the 2-port version. (You can identify NEC chipsets by the two little circles on the chip labelling.) But it's a single controller with two ports; you could just get two of them, but I realize that might be a bit of a problem on the PCI-e slot front.
I did find this thing if spare PCI-e slots is your problem, but I haven't tried it out. Odds are it won't do ACS, and mounting it neatly would be a pain, but it's awesome that it's actually available for a non-extortionate price.
I also spotted this card from Startech after some Googling (I see the circles that indicate a NEC chipset) but it ain't cheap.
I'd avoid any PCI-e x1 controllers. I've got a number of them. If you're looking to add one drive per port, then no issue. If you start to add hubs, they tend to fall over themselves.
Get one that uses a dedicated channel per port (they'll usually be a PCI-e x4). Something like this https://www.amazon.com/StarTech-com-Express-SuperSpeed-Dedicated-Channels/dp/B00HJZEA2S
I've added 13 port hubs to each port with zero issues.
This one reads 15 8TB drives for me in 36 seconds on Jminer. It has a Molex connection but seems to work pretty well without additional power. 3 powered hubs attached.
4 Port PCI Express (PCIe) SuperSpeed USB 3.0 Card Adapter w/ 4 Dedicated 5Gbps Channels - UASP - SATA / LP4 Power https://www.amazon.com/dp/B00HJZEA2S/ref=cm_sw_r_cp_apip_u5W5uiQEv2aZ6